However, when creating a new my material, render the open scene file and set the result to the material preview image.

When I set the MATPREVIEW_GENERATE_IMAGE renderer to Software, Standart, or Physical etc, I get a material preview the sphere in MATPREVIEW_MODIFY_CACHE_SCENE.

The issue is that my renderer does not render MATPREVIEW_MODIFY_CACHE_SCENE, but renders the scene I am editing and returns it to the material preview.
I maybe that the description of the custom renderer itself is wrong.
How to call, conditions of VideoPost::Exexcute, etc ...

With reference to the issue reported and considering the code shared, I suppose the issue is likely to be related to what your rendering engine receives (in terms of scene) when attempting to generate a material preview. What scene actually are you passing to your render in the videopost data? Have you tried to close the active document and try to generate the material preview?